The present invention relates to an aluminum material which is useful for the particle accelerating pipes (beam lines) of accelerators such as synchrotrons and also for other vacuum apparatus such as film producing apparatus, surface analyzing apparatus and nuclear fusion apparatus.
The term "aluminum" as used herein and in the appended claims includes pure aluminum and all aluminum alloys. Further the term "inert gas" includes argon gas and helium gas on the Periodic Table and also nitrogen gas which is inert to aluminum.
While stainless steel has been chiefly used for producing particle accelerating pipes, aluminum has been found suitable for this application recently and placed into use, because as compared with stainless steel, aluminum is less likely to produce induced radioactivity which, even if emitted, decays more rapidly, and further because aluminum has higher thermal and electrical conductivities and is lower in the gas release rate of the surface, more lightweight and and better in workability. The interior of the particle accelerating pipe must be maintained in a high vacuum in order to pass particles at a high velocity. Accordingly the performance of the pipe is dependent on how to maintain a high vacuum within the pipe. Furthermore, although stainless steel was used also for the lining plate of the vaccum chamber of the above-mentioned film producing apparatus, aluminum has been introduced into use for the plate because aluminum is lower in the gas release rate of its surface, more lightweight and better in workability. It is also necessary to maintain a high vacuum within the vacuum chamber of the apparatus in order to produce a film therein free from impurities. The vacuum portions of other vacuum apparatus also require a high vacuum as desired.
To produce a high vacuum within the particle accelerating pipe or the vacuum portion of other vacuum apparatus, it is conventional practice to degrease with an organic solvent or the like the inner surface of the pipe or the surface of aluminum material facing the vacuum portion and thereafter heating the surface at about 150.degree. C. for about 24 hours for degassing, in combination with discharge cleaning in hydrogen gas, argon gas or the like when so desired. Such a procedure, however, requires a long period of time, is inefficient and is not fully satisfactory in creating a high vacuum within the vacuum portion.
In order to maintain a high vacuum within the vacuum portion of the vacuum apparatus, it is important to reduce the amount of gas to be released from the surface of aluminum material as a product and facing the vacuum portion. In this respect, we carried out experiments and research and conceived that the state of the coating on the surface of aluminum material greatly influences the degree of vacuum.
As is well known, aluminum is a metal which is very prone to oxidation, and an oxide coating is formed over the surface when it is brought into contact with oxygen. Further when aluminum is allowed to stand in the presence of water or moisture, a hydrated oxide coating is formed over the surface. The hydrated oxide coating grows more remarkably if the temperature of the hydrated oxide coating forming reaction is higher. In an environment of high temperature, a coating of hydrated oxide, such as boemite (quasi-boemite) or bialite, is formed on the surface of aluminum. Unlike the aluminum oxide coating which is formed in the absence of water, such a hydrated oxide coating is very coarse and porous and has a complexly intricate texture. Additionally, the coating has a large thickness.
The aluminum material produced by the usual extrusion process has a hydrated oxide surface coating which is formed by the contact of aluminum with water-containing atmosphere (oxygen) during extrusion. During the production process, moreover, the aluminum material is exposed to a high temperature, which accelerates the hydrated oxide coating forming reaction to form a coating of large thickness. The thick hydrated oxide coating is porous as mentioned above and therefore adsorbs a large amount of water. Further after the production of the aluminum material, the coating, which is not compact, adsorbs vacuum reducing substances, such as water, hydrocarbons, carbon dioxide and carbon monoxide, which are present in the atmosphere. Since the coating has the above characteristics, these substances become incorporated into the coating. Consequently, it becomes difficult to remove the substances even when the coating is exposed to a vacuum. The presence of these substances appear responsible for the difficulty encountered in giving an improved degree of vacuum with use of aluminum material. Further, to give enhanced mechanical strength to the aluminum material formed, the material is heated at a high temperature and then cooled with water and air for hardening or heat treatment. During this treatment, the hydrated oxide coating formed during the production process further grows, while the vacuum reducing substances already adsorbed become incorporated into the coating.
The plate and foil materials which are formed by rolling have adhered to the surface the rolling oil which is a vacuum reducing substance and further have a porous hydrated oxide coating formed during annealing. Press-formed products also have adhered thereto a processing oil which is a vacuum reducing substance.
Aluminum materials for vacuum apparatus include a hollow aluminum extrusion material useful for particle accelerating pipes for synchrotron and like accelerators. We have already proposed a process for producing such aluminum extrusion material which process is characterized in that the inner surface of the material is held out of contact with air (Published Examined Japanese Patent Application No. SHO 59-19769). Although suited to hollow extrusion materials, this process is not applicable to the plate, foil and like materials to be produced by rolling, press-formed products and solid extrusion materials.